Ceramic dielectrics



March 10, 1970 s mo u FUJUNARA ETAL 3,499,772

CERAMIC DIELECTRICS Filed Dec. 29, 1967 2 Sheets-Sheet 1 Bi O -2TiOCqTiOa INVENTOIS SHINOBU FUJIWARA K IYOTO KAWAKAMI ATTORNEYS March 10,1970 SHINQBU FUJIWARA ETAL 3,499,772

CERAMIC DIELECTRICS Filed Dec. 29, 1967 a 2 Sheets-Sheet 2 SrTi O 3 Bi O-2TiO K 20 CqTiO -|2oox 10/ aizogzrioa a0 60 40 2o cQ i KIYOTO KAWAKAMIFIG.3

BY V

ATTORNEYS United States Patent C) US. Cl. 10639 1 Claim ABSTRACT OF THEDISCLOSURE Ceramic dielectric materials are provided which have highdielectric constants, low temperature coeflicients and high Q values.The materials consist essentially of a sintered mixture of 6 to 80% byweight of SrTiO to 60% by weight of CaTiO and 15 to 34% by weight of BiO -2TiO This is a continuation-in-part application of copendingapplication, Ser. No. 359,131, filed Apr. 13, 1964, now abandoned.

This invention relates to improvements in ceramic dielectrics.

An object of the invention is to provide ceramic dielectrics which haverelatively high dielectric constants, lower temperature coefficients andhigh Q-values, and which show excellent characteristics when they areapplied in the various communication equipment.

As a ferroelectric material, barium titanate (BaTiOg) has hitherto beenwell known to be of practical use over a wide electrical field, but itsapplication to the electrical devices has greatly been limited by theexistence of the maximum of its dielectric constant at about 120 C., andby the existence of the transition point at about 0 C.

Since BaTiO has the property of easily forming a solid solution withisomorphic compounds, the addition of calcium titanate (OaTiO magnesiumtitanate (MgTi0 and other titanates to BaTiO has been tried to obtain asmall temperature coefiicient. Although the temperature dependence ofthe dielectric constant of BaTiO is improved by the addition of thesetitanates, the dielectric constant of the body is much lowered resultingin the loss of its practical use.

Other high dielectric materials have also had the similar shortcomingsof large temperature coefiicients and low Q-values.

The object of the present invention is to provide ceramic dielectricswhich are free of the above mentioned shortcomings of other ceramicdielectrics, and which are composed mainly of strontium titanate,calcium titanate, and a mixture of bismuth oxide and titanium oxide,(S1TiO CaTiO and Bi O -2TiO to obtain a high dielectric constant and asmall temperature coefficient.

In this invention, in order to obtain the most successful results, thecomposition of the above three ingredients, according to the detailedexperimental discussion, should be limited to the ranges given below:

Percent by wt.

When the content of strontium titanate SrTiO exceeds by weight, thetemperature coefiicient of dielectric constant of the body becomes toolarge for practical use and firing temperature is required to be higherthan 1380 C. When the content of strontium titanate SrTiO is less than6% by weight, the dielectric constant of the body is lowered to givepredominance to the characteristics of calcium titanate CaTiO itself,and the body could not be used where high dielectric constant isrequired. When the content of calcium titanate CaTiO is less than 5% byweight, the Q-value of the body is lowered and its dielectric constantdepends greatly on temperature. When the content of Bi O -2TiO exceeds34% by weight, both the dielectric constant and the Q-value .are loweredand 'the body has high porosity and vitreous surface. When the contentof Bi O -2TiO is less than 15% by weight, dielectric constant is low andfiring temperature should be as high as 1360 C.

The relations between the three ingredients of the ceramic dielectricsof the invention, i.e. strontium titanate, calcium titanate and mixtureof bismuth oxide and titanium oxide (SrTiO CaTiO and Bi O -2TiO and thedielectric constants, the temperature coefficients, and Q-values aregiven by the triaxial diagrams of FIGURES 1 to 3, where the valuesindicated refer to the dielectric constants, the temperaturecoefiicients, and the Q-values respectively, and all the values aretaken from the measurements at 1 mc./s.

In preparing the ceramic dielectrics of the invention, SrTiO and CaTiOare first finished by pre-sintering at about 1300 C. and by grinding,and then they are mixed with Bi O and TiO After the mixture is finishedby compression molding, the mixture is made up into the dielectrics bysintering at above 1000 C.

In this manner the main ingredients, SrTiO and are prefired so that lossof usefulness of the raw materials by moisture absorption during storagecan be completely eliminated, and the obtained dielectrics live toexpectations sufliciently. Examples of the ceramic dielectrics producedaccording to the invention are described below in some detail.

EXAMPLE 1 Powdered SrTiO and CaTiO each of which has been pre-sinteredat 1300 C., are mixed with Bi O -2TiO- at the following ratio:

Parts by wt. 60 10 30 EXAMPLE 2 Powered SrTiO and CaTiO which have beenpresintered in the same way as in Example 2, are mixed with Bi 0 -2TiOat the following ratio:

srrio CaTiO Parts by Wt. 40 40 20 SrTiO CaTiO Bi O After molding, themixture is sintered at 1350 C. to make a dielectric. The body shows thefollowing characteristics: the dielectric constant=357,

the temperature coefficient=1826 10- deg? and QESSOO.

As described above, the application provides ceramic dielectrics whichhave higher dielectric constant and smaller temperature coefficient incomparison with those hitherto used. Ceramic dielectrics of theapplication have been applied for practical uses, e.g., as capacitorelements for television tuning circuits and as rotor parts of variableceramic capacitors, and were found to function excellently. Moreover,dielectrics of the application possess low porosity and dense structure,and this enables them to be used reliably as capacitors under conditionsof severe temperature variations or high humidity. Such excellentcharacteristics were first exhibited by the body of the application.

What is claimed is:

1. A ceramic dielectric consisting essentially of a sintered mixture of40 parts by weight of SrTiO 40 parts by Weight of CaTiO and 20 parts byweight of Bl203 References Cited UNITED STATES PATENTS 12/1962 Domanskil0639 3,179,525 4/1965 Welsby et a]. l0639 HELEN M. MCCARTHY, PrimaryExaminer US. Cl. X.R.

